Assuming that the client uses FileMaker Pro or Go: the schema is loaded so that it can be executed. The amount of data depends on how the file opens. If you open on a blank layout of a table with no records then there is virtually no data that is loaded. Otherwise FM will load the data to make the current layout work, plus some extra records to anticipate the user navigating. So the most expensive 'opening' would be on a list layout showing a great number of visible records.

(or a form view with many portals showing lots of records in the portals).

That loading would be made worse if any of this shows summary or unstored calculation fields because the client would have to calculate the values before it can display them.

It's unlikely that you'll stress the memory on the machine though, unless it really is marginal. FMP keeps its cache on disk for the most part. So if the client is running low on disk space, it will stress the processors and the disk i/o before it stresses the RAM.